The allocation of spectrum resources in a wireless communication system needs to be carefully managed, particularly in a cellular communication network. An example of such a network operates according to one or more of the 3GPP standards (Third Generation Partnership Project). A particular standard may also be referred to as a “radio access technology”. In such systems, a plurality of carrier frequency bands (“carriers”) are made available for communication between devices. In the case of a cellular network an allocated carrier can be used to communicate on the uplink and downlink between a mobile user terminal and a base station (also called a “Node-B” in 3 G terminology). For example in the latest LTE (Long Term Evolution) standards of 3GPP there are defined bands 1 to 17 as will be familiar to a person skilled in the art. 3GPP also covers LTE Advanced standards or “4 G”.
Within a given carrier, one or more mobile user terminals can communicate uplink and/or downlink signals on the allocated carrier. Multiple such signals can be multiplexed onto the same carrier using one or more of: time division multiplexing whereby each signal is given a different respective time slot, code division multiplexing whereby each signal is spread by a different respective spreading code, and/or orthogonal frequency division multiplexing whereby a carrier is divided into a plurality of subcarriers.
Aside from this, some modern standards also allow for a technique known as carrier aggregation, whereby a given device is enabled to communicate using an aggregate of multiple carriers (not just subcarriers, but multiple carriers, typically being the largest unit of frequency band used in the radio access technology in question). The component carriers forming the aggregate may be contiguous or may not. For example carrier aggregation is enabled in LTE Advanced, but not regular LTE standards.
In order to make use of the aggregated carrier, a device is configured according to a suitably advanced radio access technology. Typically a user terminal configured according to an earlier radio access technology will only be able to access each carrier band as a separate, individual band, and may only be allocated one single carrier for use at any one time. A user terminal configured according to a later radio access technology on the other hand may be enabled to aggregate two or more carriers for use together simultaneously. For example, to an LTE terminal, each component carrier appears as a separate, individual LTE carrier and may only be used one at a time; while an LTE-Advanced terminal can exploit the total aggregated bandwidth of the two or more component carriers. Using LTE Advanced carrier aggregation, it is possible to utilise more than one carrier and in this way increase the overall transmission bandwidth.
However, interference can occur between the bands of the aggregated carrier scheme.
For example one particular problem occurs with the receiver sensitivity degradation in an LTE carrier aggregation (CA) system where band 4 (2100 MHz) and band 17 (700 MHz) are the two frequency bands that are used. This is particularly important for North America. The problem is that the band 17 uplink generates third-harmonic products that fall into the band 4 downlink. As a result, particularly at high transmit powers, the sensitivity of the band 4 receiver is degraded.
Attempts have been made to suppress this harmonic interference using hardware filters. However, this “brute force” approach has met with limited success. For example the designer tries to suppress the interference at one location only to find it reappear at another. Given the difficulties with such interference, proposals have been made to circumvent the problem by placing constraints on maximum power output and relaxing sensitivity requirements. Reference is made to 3GPP publication R4-120744 (3GPP TSG RAN WG4 Meeting #63, Dreseden, Germany, 6-10 Feb. 2012, agenda item 6.10.1, “MOP and REFSENS Relaxation for Carrier Aggregation for Band 4 and 17).